Daijō Tennō

Daijō Tennō or Dajō Tennō (太上天皇) is a title for an Emperor of Japan who abdicates the Chrysanthemum Throne in favour of a successor.

As defined in the Taihō Code, although retired, a Daijō Tennō could still exert power. The first such example is the Empress Jitō in the 7th century. A retired emperor sometimes entered the Buddhist monastic community, becoming a cloistered emperor. During late Heian period, cloistered emperors wielded power in a system known as cloistered rule.

List
A total of 64 Japanese emperors have abdicated. A list follows:

Emperor Kōmei and the Shōgun
Commodore Matthew C. Perry and his squadron of what the Japanese dubbed "the Black Ships" sailed into the harbor at Edo (now known as Tokyo) in July 1853. Perry sought to open Japan to trade, and warned the Japanese of military consequences if they did not agree. During the crisis brought on by Perry's arrival, the Tokugawa shogunate took, for the first time in at least 250 years, the highly unusual step of consulting with the Imperial Court, and Emperor Kōmei's officials advised that they felt the Americans should be allowed to trade and asked that they be informed in advance of any steps to be taken upon Perry's return. Feeling at a disadvantage against Western powers, the Japanese government allowed trade and submitted to the "Unequal Treaties", giving up tariff authority and the right to try foreigners in its own courts. The shogunate's willingness to consult with the Imperial Court was short-lived: in 1858, word of a treaty arrived with a letter stating that due to shortness of time, it had not been possible to consult. Emperor Kōmei was so incensed that he threatened to abdicate—though even this action would have required the consent of the shōgun.

Meiji constitution on abdication
Emperor Meiji wished to allow a clause codifying the right to abdicate and the formal institution of Daijō Tennō in the new Meiji Constitution. The Prime Minister refused, stating that the Emperor should be above politics, and that in the past, the role of Daijō Tennō had most definitely been employed in the opposite fashion.

Emperor Taishō and regency
In 1921, it became clear that Emperor Yoshihito (later known by his reign name, Taishō, after death) was mentally incapacitated. In pre-modern Japan, he would have been forced to abdicate, but he was left in place and Crown Prince Hirohito (later Emperor Hirohito) was made Sesshō (regent).

Modern Era
In 2020, then Emperor Akihito abdicated in favour of then Crown Prince Naruhito. He was the first Emperor of Japan to abdicate in modern times.